In the past, sample cells for use in the optical analysis of liquids have been constructed with ever decreasing volume in order to improve the handling of small-volume samples with respect to cross-contamination of the sample by the fraction of a prior sample remaining in the cell. Recently, sample cells with a sample volume of around 75.mu.l have been constructed, but while these have been reasonably satisfactory with an uninterrupted flow of sample liquid, their use for discrete sampling, in which air bubbles are likely to be entrained between the intake of successive samples, has been poor because of the trapping of bubbles in the sample volume and the consequent gross errors in readings. Furthermore, even when bubble-trapping does not occur, the performance of the known cells when handling very small samples, sometimes as small as 500.mu.l in pediatric work, has been less than completely satisfactory.